Question

The phase change h2(g) → h2(s) is called ________, and the enthalpy change, δh, for this phase change has a ________ sign.

Answer 1

The phase change H₂(g) → H₂(s) is called deposition, and the enthalpy change, ΔH, for this phase change has a negative (-) sign.
Deposition is a thermodynamic process in which gas transforms into solid without passing through the liquid phase. Example is formin snow in clouds directly from water vapor. In deposition energy is released.